The present invention relates to a flow rate sensor, a flow rate measuring device, and a flow rate control device for measuring a flow rate of a comparatively small flow rate of fluid such as gas.
In general, when producing a semiconductor product such as a semiconductor integrated circuit, for example, CVD film formation and etching are repeatedly performed in various semiconductor manufacturing devices. In this case, it is necessary to accurately control the very small processing gas flow rate. For example, a flow rate control device such as a mass flow controller is used.
This type of flow rate control device can accurately control a very small flow rate (mass flow rate) of fluid such as gas. However, the range of the accurately controllable flow rate is comparatively small and is decided during a manufacturing stage by design. For example, a flow rate control device designed for accurately controlling flow rate in a small flow rate range such as 0 to 5 sccm cannot be used for a range of, for example, 100 sccm. On the contrary, a flow rate control device designed for accurately controlling the flow rate of a large flow rate range such as 100 sccm cannot function correctly in the small flow rate range such as 5 sccm.
Accordingly, when using the aforementioned flow rate control device in a gas piping of a semiconductor manufacturing device, it is necessary to select a flow rate control device designed to function accurately in a flow rate range corresponding to the gas flow rate in the piping as is disclosed, for example, in U.S. Pat. No. 5,410,912 and JP-A-1-227016.
Here, explanation will be given on configuration of an ordinary flow rate control device with reference to FIG. 17 and FIG. 18. FIG. 17 shows configuration of a flow rate control device arranged on a gas piping and FIG. 18 shows a flow rate sensor of the flow rate control device.
As shown in the figures, this flow rate control device 2 is arranged in the middle of a gas piping 4, for example, so as to regulate its flow rate. The flow rate control device 2 has a fluid passage 6 made from, for example, stainless steel and its both ends are connected to the gas piping 4. The flow rate control device 2 has a flow sensor 5 positioned at the front stage of the flow passage 6 and a flow rate control system 7 positioned at the rear stage.
Firstly, the flow rate sensor 5 has a bypass 8 arranged at the upstream side of the gas fluid flow direction of the flow passage 6 for flowing most of the flow amount. The both ends of the bypass 8 are connected to a sensor pipe 14 taking a roundabout route, so that a small amount of gas fluid as compared to the bypass 8 flows at a constant ratio. A pair of resistors R1 and R4 connected in series is wound around the sensor pipe 14, so as to obtain a detection value (potential difference) Vs by a sensor circuit 16 connected to this.
This detection value Vs is supplied to a flow rate control unit 18 composed of a microcomputer or the like, for example. According to the detection value Vs, the currently flowing gas rate is calculated and the fluid control system 7 is controlled so that the flow rate is matched with an instruction value S1 supplied from outside.
This fluid control system has a flow rate control valve 12 arranged at the downstream side of the fluid passage 6. This flow rate control valve 12 has diaphragm, for example, as a valve body for directly controlling the flow rate of the gas fluid. The diaphragm 10 can adjust its valve open degree by an actuator 20 composed of, for example, a laminated piezoelectric element. In response to a signal from the flow rate control unit 18, the actuator 20 is operated by a drive signal output from a drive unit 22.
FIG. 18 shows the relationship between the resistors R1, R4 and the sensor circuit 16. That is, with respect to the resistors R1 and R4 connected in series, a circuit of two reference resistors R5 and R6 connected in series is connected in parallel, thereby forming a so-called bridge circuit. A constant current source 24 is connected to this bridge circuit so as to flow a constant current. Moreover, a connection point between the resistors R1 and R4 and a connection point between the reference resistors R5 and R6 are connected to the input side and a differential circuit 26 is provided. A difference between the potentials of the two connection points is obtained and this potential difference is output as the detection value Vs.
Here, the aforementioned resistors R1 and R4 are made from a material changing its resistance value according to the temperature and its heat value according to the current. The resistor R1 is wound at the upstream side of the gas flow direction and the resistor R4 is wound at the downstream side. Moreover, the reference resistors R5 and R6 are maintained substantially at a constant temperature.
In the flow rate control device 2 having the aforementioned configuration, when no gas fluid is flowing in the sensor pipe 14, the temperature values of the resistors R1 and R4 are identical. Accordingly, the bridge circuit is balanced and the potential difference as a detection value of the differential circuit 26 is, for example, zero.
When it is assumed that a gas fluid flows in the sensor pipe 14 with a flow rate Q, the gas fluid is heated by the heat value of the resistor R1 located at the upstream side and flows as it is to the position of the resistor R4 at the downstream side. As a result, heat movement is caused, resulting in a temperature difference between the resistors R1 and R4, i.e., resistance difference between the resistors R1 and R4. The potential difference generated here is substantially proportional to the gas flow rate. Accordingly, by multiplying this detection value Vs by a predetermined gain, it is possible to obtain the gas flow rate then. Moreover, the valve open degree of the flow rate control valve 12 is controlled so that the gas flow rate detected is matched with the instruction value S1 (actually, voltage value). Moreover, the relationship between the gas flow rate and the potential difference as the detection value has excellent proportional linearity at first and in a range that can be used for flow rate control.
However, as the potential difference increases, saturation is caused and the range cannot be used for flow rate control. Accordingly, various types of flow rate control devices are prepared by modifying the gain value to be multiplied to the detection value Vs and the resistance values of the resistors R1 and R4, so that flow rate control devices having different ranges for appropriately measuring the gas flow rate are available.
Next, explanation will be given on a semiconductor manufacturing device using the flow rate control device having the aforementioned configuration with reference to FIG. 19. Here, an example is given for supplying the same gas at different flow rates.
As shown in the figure, this semiconductor manufacturing device has a treatment chamber 30 capable of being vacuumed. A gas source 32 is connected to the treatment chamber 30 via a gas pipe 4. This gas pipe 4 is branched to two different routes of gas pipes 4A and 4B. The gas pipe 4A and 4B have flow rate control devices 2A, 2B and open/close valves 34, 36, respectively. For example, according to an instruction from an device control unit 38 composed of a microcomputer, the flow rate control devices 2A and 2B are selectively operated. For example, one of the flow rate control devices 2A is set for a large flow rate and the other flow rate control device 2B is set for a small flow rate. Accordingly, as shown in FIG. 19, when gas is to be flown at a large flow rate at the beginning of the treatment and after this, gas is to be flown at a small flow rate, control is made sop that the flow rate control device 2A for a large flow rate is operated firstly and after this the flow rate control device 2B for a small flow rate is operated.
As has been explained with reference to FIG. 19 and FIG. 20, in the flow rate control device having the aforementioned configuration, when the same gas should be supplied in ranges of greatly different flow rates, a plurality of flow rate control devices 2A, 2B designed for the different ranges of gas flow rate should be used. This increases the facility cost. Moreover, when arranging flow rate control devices of different flow rate ranges, it is necessary to have a corresponding space and it is difficult to provide an additional device.
Moreover, in the structure of the gas pipe 4 shown in FIG. 19, when switching between the gas flow rates, it is necessary to perform open/close switching of the open/close valves 34, 36. Here, in order to prevent a sudden gas flow stop or a sudden flow start, the valve open/close switching operation requires a certain time, which increases the wafer treatment time, thereby lowering the throughput.